NYC Auto Mechanic (Diesel) (DCAS) Practice
Exam Question and Answers with Rationales
Latest Version
Question 1
What is the primary purpose of the turbocharger on a modern diesel engine?
A. To reduce engine oil temperature
B. To increase the amount of air entering the cylinders
C. To reduce fuel pressure
D. To eliminate the need for an intercooler
Answer: B. To increase the amount of air entering the cylinders
Rationale: A turbocharger uses exhaust-gas energy to drive a compressor that
forces additional air into the engine. More available oxygen allows the engine to
burn more fuel efficiently and produce greater power. An intercooler may be used
to cool the compressed intake air, but it is not eliminated by the turbocharger.
Question 2
A diesel engine cranks normally but will not start. Fuel supply to the injection
system is confirmed. Which condition should be checked next?
A. Engine compression
B. Tire pressure
C. Brake-fluid level
D. Differential lubricant
Answer: A. Engine compression
Rationale: A diesel engine requires adequate compression temperature for
ignition. If fuel is available but the engine does not start, insufficient compression,
incorrect injection timing, lack of sufficient cranking speed, or an electrical/control
problem should be investigated. Compression testing is an important diagnostic
step.
,Question 3
What is the primary function of a diesel fuel injector?
A. Compress intake air
B. Atomize and deliver fuel into the combustion chamber
C. Remove water from engine oil
D. Regulate coolant pressure
Answer: B. Atomize and deliver fuel into the combustion chamber
Rationale: The injector delivers a precisely metered quantity of highly pressurized
diesel fuel and breaks it into a fine spray. Proper atomization and injection timing
are essential for efficient combustion.
Question 4
A diesel engine produces excessive black smoke under acceleration. What is the
most likely general cause?
A. Excessive oxygen in the combustion chamber
B. Too much fuel or insufficient air
C. Excessive coolant circulation
D. Low windshield-washer fluid
Answer: B. Too much fuel or insufficient air
Rationale: Black smoke generally indicates incomplete combustion caused by an
overly rich air-fuel condition. Common causes include restricted air intake, a
malfunctioning turbocharger, excessive fueling, dirty injectors, or problems with
air-management systems.
Question 5
What is the purpose of the diesel particulate filter (DPF)?
A. To remove particulate matter from exhaust gases
B. To increase transmission pressure
C. To cool engine coolant
D. To filter hydraulic fluid
,Answer: A. To remove particulate matter from exhaust gases
Rationale: A DPF traps soot and particulate matter from diesel exhaust. The
accumulated soot must periodically be oxidized through a process known as
regeneration.
Question 6
During DPF regeneration, what happens to the accumulated soot?
A. It is converted primarily into gaseous products through oxidation
B. It is transferred into the engine oil
C. It is pumped into the fuel tank
D. It is stored permanently in the exhaust manifold
Answer: A. It is converted primarily into gaseous products through oxidation
Rationale: Regeneration raises exhaust temperatures sufficiently to oxidize
accumulated soot. Depending on the system, regeneration can occur passively or
through active strategies that increase exhaust temperature.
Question 7
Which instrument is most appropriate for measuring electrical resistance?
A. Ammeter
B. Voltmeter
C. Ohmmeter
D. Tachometer
Answer: C. Ohmmeter
Rationale: An ohmmeter measures resistance in ohms. Resistance testing should
generally be performed with the circuit de-energized unless the manufacturer's
diagnostic procedure specifically states otherwise.
, Question 8
A diesel vehicle's battery voltage drops significantly while cranking, and the starter
turns slowly. What should be checked first?
A. Battery condition and cable connections
B. Differential gear ratio
C. Brake shoe thickness
D. Exhaust temperature sensor only
Answer: A. Battery condition and cable connections
Rationale: Diesel engines require substantial cranking power because of their high
compression ratios. A weak battery, corroded terminals, loose connections, or
excessive cable resistance can cause slow cranking and prevent starting.
Question 9
What is the purpose of glow plugs in many diesel engines?
A. To heat the engine oil continuously
B. To assist cold starting by heating the combustion area
C. To increase transmission pressure
D. To cool the exhaust manifold
Answer: B. To assist cold starting by heating the combustion area
Rationale: Glow plugs provide additional heat during cold starting. They are
especially important when cylinder temperatures are insufficient to support reliable
compression ignition.
Question 10
A diesel engine has difficulty starting only when cold but starts normally once
warm. Which system is particularly important to inspect?
A. Glow-plug/preheat system
B. Parking-brake cable
C. Differential breather
D. Wheel alignment
Exam Question and Answers with Rationales
Latest Version
Question 1
What is the primary purpose of the turbocharger on a modern diesel engine?
A. To reduce engine oil temperature
B. To increase the amount of air entering the cylinders
C. To reduce fuel pressure
D. To eliminate the need for an intercooler
Answer: B. To increase the amount of air entering the cylinders
Rationale: A turbocharger uses exhaust-gas energy to drive a compressor that
forces additional air into the engine. More available oxygen allows the engine to
burn more fuel efficiently and produce greater power. An intercooler may be used
to cool the compressed intake air, but it is not eliminated by the turbocharger.
Question 2
A diesel engine cranks normally but will not start. Fuel supply to the injection
system is confirmed. Which condition should be checked next?
A. Engine compression
B. Tire pressure
C. Brake-fluid level
D. Differential lubricant
Answer: A. Engine compression
Rationale: A diesel engine requires adequate compression temperature for
ignition. If fuel is available but the engine does not start, insufficient compression,
incorrect injection timing, lack of sufficient cranking speed, or an electrical/control
problem should be investigated. Compression testing is an important diagnostic
step.
,Question 3
What is the primary function of a diesel fuel injector?
A. Compress intake air
B. Atomize and deliver fuel into the combustion chamber
C. Remove water from engine oil
D. Regulate coolant pressure
Answer: B. Atomize and deliver fuel into the combustion chamber
Rationale: The injector delivers a precisely metered quantity of highly pressurized
diesel fuel and breaks it into a fine spray. Proper atomization and injection timing
are essential for efficient combustion.
Question 4
A diesel engine produces excessive black smoke under acceleration. What is the
most likely general cause?
A. Excessive oxygen in the combustion chamber
B. Too much fuel or insufficient air
C. Excessive coolant circulation
D. Low windshield-washer fluid
Answer: B. Too much fuel or insufficient air
Rationale: Black smoke generally indicates incomplete combustion caused by an
overly rich air-fuel condition. Common causes include restricted air intake, a
malfunctioning turbocharger, excessive fueling, dirty injectors, or problems with
air-management systems.
Question 5
What is the purpose of the diesel particulate filter (DPF)?
A. To remove particulate matter from exhaust gases
B. To increase transmission pressure
C. To cool engine coolant
D. To filter hydraulic fluid
,Answer: A. To remove particulate matter from exhaust gases
Rationale: A DPF traps soot and particulate matter from diesel exhaust. The
accumulated soot must periodically be oxidized through a process known as
regeneration.
Question 6
During DPF regeneration, what happens to the accumulated soot?
A. It is converted primarily into gaseous products through oxidation
B. It is transferred into the engine oil
C. It is pumped into the fuel tank
D. It is stored permanently in the exhaust manifold
Answer: A. It is converted primarily into gaseous products through oxidation
Rationale: Regeneration raises exhaust temperatures sufficiently to oxidize
accumulated soot. Depending on the system, regeneration can occur passively or
through active strategies that increase exhaust temperature.
Question 7
Which instrument is most appropriate for measuring electrical resistance?
A. Ammeter
B. Voltmeter
C. Ohmmeter
D. Tachometer
Answer: C. Ohmmeter
Rationale: An ohmmeter measures resistance in ohms. Resistance testing should
generally be performed with the circuit de-energized unless the manufacturer's
diagnostic procedure specifically states otherwise.
, Question 8
A diesel vehicle's battery voltage drops significantly while cranking, and the starter
turns slowly. What should be checked first?
A. Battery condition and cable connections
B. Differential gear ratio
C. Brake shoe thickness
D. Exhaust temperature sensor only
Answer: A. Battery condition and cable connections
Rationale: Diesel engines require substantial cranking power because of their high
compression ratios. A weak battery, corroded terminals, loose connections, or
excessive cable resistance can cause slow cranking and prevent starting.
Question 9
What is the purpose of glow plugs in many diesel engines?
A. To heat the engine oil continuously
B. To assist cold starting by heating the combustion area
C. To increase transmission pressure
D. To cool the exhaust manifold
Answer: B. To assist cold starting by heating the combustion area
Rationale: Glow plugs provide additional heat during cold starting. They are
especially important when cylinder temperatures are insufficient to support reliable
compression ignition.
Question 10
A diesel engine has difficulty starting only when cold but starts normally once
warm. Which system is particularly important to inspect?
A. Glow-plug/preheat system
B. Parking-brake cable
C. Differential breather
D. Wheel alignment
